Increasingly stringent emissions regulations require automobile manufacturers to develop comprehensive on-board diagnostic (OBD) systems for exhaust gas monitoring. Compact, inexpensive sensors are particularly in demand for monitoring and control of regulated pollutants including hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx). Sensors for these applications have been proposed based on semiconducting oxides, heterocontacts in semiconducting oxides, surface acoustic waves, and capacitance. Other sensors are based on solid-state electrochemical devices, which typically use a solid ceramic electrolyte attached with two or more metal or metal-oxide electrodes and operate in either potentiometric (open circuit) or amperometric (DC-biased) modes.
Significant progress has been made towards the development of deployable sensors using yttria-stabilized zirconia (YSZ) as the electrolyte of solid-state electrochemical devices. However, there still exist significant shortcomings related to stability, sensitivity, response time, and cross-sensitivity. These shortcomings are responsible for only one type of NOx sensor being available commercially to date. The commercially available amperometric NOx sensors are not ideal for widespread use due to high cost, complexity, and limited performance. The benchmark sensor is the well-known YSZ-based oxygen sensor currently used in almost all automobiles. Although that sensor demonstrates the commercial feasibility of this technology, it addresses a task that is less complicated than low concentration (ppm level) gas sensing used in NOx applications.
Some approaches for solid-state electrochemical sensors use frequency-domain impedancemetric modes of operation. This approach relies on specific material compositions and microstructures to maximize the sensor response at higher frequencies, because higher frequencies allow for faster sampling rates and improved signal-to-noise ratios. This approach also relies on the measurement of the phase angle as a metric for monitoring the gas concentration.